The present invention relates to a thin-layer laminate having utility as a getter for undesired gaseous contaminants in a vacuum enclosure, and to a vacuum enclosure characterized by the presence of this getter.
Without prejudice to the broad concept and scope of the invention, the background will be described with particular reference to high vacuum devices in the form of Infra-red (IR) radiation detectors. These commonly comprise a dewar envelope having an inner wall and an outer wall, a vacuum space being present between the inner and outer walls; the inner wall defines an inner chamber of the dewar; an infra-red radiation detector element is mounted in the vacuum space and on the end face of said inner wall; a cooling element is provided in the inner chamber and serves to cool the inner wall and the detector element mounted thereon during operation of the detector. The cooled inner wall is often termed “the cold finger” of the detector.
It is known that a prime cause of detector failure is the gradual degradation of the vacuum in the space between the inner and outer walls due to diffusion and internal out-gassing of the various parts of the detector. The excessive out-gassing which occurs in infrared detectors is associated with the fact that the gases cannot be driven out by baking the whole device during pumping (in the way which is usual for other vacuum devices) because infrared detector elements are damaged at high temperatures. Hydrogen is the most destructive gas for semiconductor devices (gallium arsenide and/or indium phosphide), most thermo-conductive gas and besides it is also the most difficult gas component to remove. It has been established that hydrogen is the primary cause of performance degradation in these devices in RF, AC, and DC operating modes.
This degradation in the vacuum eventually leads to the situation in which the cooling element is no longer able (at least in an efficient manner) to cool the detector element sufficiently fast to the desired temperature for efficient detection of infra-red radiation. Thus, the detector lifetime is curtailed, especially as only limited cooling power is available for infrared detectors. Furthermore, the out-gassing into the vacuum space provides a thermal path between the cold finger and the outside of the detector possibly leading to the formation of dew on the infrared window of the detector in a normal atmosphere.
In order to reduce the effects caused by this internal out-gassing at least one getter is normally provided in the vacuum space for removal of undesired gas molecules from this space.
Usually an infrared detector incorporates a non-evaporable chemically-active SAES getter (e.g. SAES Getters Inc., St 171-St 172 Brochure;) which is mounted on the outer wall and in the vacuum space between the outer wall and the cold finger. Such chemical getters are typically activated by heating to a high temperature after evacuation and sealing of the dewar envelope. This is normally achieved with an electrical heating element embedded in the getter material formed as a unit with electrical connection leads passing through vacuum-tight seals in the dewar wall (C. Taylor, S. Whicker, “Thermal Energy Receiver”, U.S. Pat. No. 3,851,173). Such getters, when mounted on the outer envelope, require minimal spacing from the detector elements which could otherwise be damaged by the very high activation temperature. In some cases extra heat shields are used to protect the internal components from being damaged during this getter activation process. Though this type of architecture is effective in removing the residual gas molecules from the volume, it often leads to an increased size for the dewar envelope and even adoption of unconventional dewar envelopes. In addition the high temperatures reached during getter activation are also the source for additional internal out-gassing as a result of the unavoidable heating of the internal components.
These limitations of current designs are constantly driving developments seeking alternative solutions in getter applications, that would allow for a more simplified construction and a reduced activation temperature inside the detector enclosure.
The getter is typically a reactive solid material that either adsorbs, absorbs, chemisorbs, or catalyzes a reaction that immobilizes or destroys one or more targeted contaminant compounds, in particular contaminant gases. For example, hydrogen can be released from various sources within an enclosure containing electronic assemblies and subassemblies. Hydrogen does not readily escape from environmentally sealed enclosures and reacts with hydrogen sensitive components. Furthermore, the unique thermal properties of hydrogen can be the cause of increased thermal loss. Several metals and nonmetals, used in production of IR detector components, can contain dissolved hydrogen that is released over time. The package materials can also sometimes release hydrogen. Plated nickel layers used as a barrier layer for gold plating operations and plastic resins are known to release hydrogen in amounts that can degrade the vacuum level and performance of semi-conductor and electrical components (R. Ramesham, “Getters for Reliable Hermetic Packages”, JPL Publication D-1792/NASA, pp 14-17, 1999).
As regards other undesired contaminants, e.g., glass (quartz elements of the device) often contains inside a certain quantity of water, and this component is gradually released as water vapor in the device volume. Organic compounds are often responsible for the presence of water vapor, carbon monoxide CO, and carbon dioxide CO2 within the sealed device. Additional gas mixtures may be generated during device manufacturing procedures such as the out-gassing process, welding, and high-rate heating of the pumping tube, when it is disconnected from vacuum pump system.
U.S. Pat. No. 5,365,742 (Boffito, et al.) describes a device for the removal of hydrogen from a vacuum enclosure at cryogenic temperatures, which comprises a metal support (e.g. an Al strip) a composition able to absorb hydrogen, adherent to at least one surface of the support. The composition comprises a porous absorber of water vapor, preferably powdered alumina in contact with palladium oxide PdO which preferably covers, at least partially, the water absorber. In practice Pd(OH)2 mixed with alumina may be precipitated on and attached to the support, then reduced to Pd metal, and re-oxidized to PdO.
U.S. Pat. No. 5,888,925 (Smith et al.) discloses a hydrogen absorbing material, and a method for its manufacture wherein platinum group metal oxide(s), a desiccant (such as a molecular sieve) and a matrix-forming binder (such as an RTV silicone) are mixed together; and the mixture is cured in an oxygen-containing gas (e.g. air) for a time (e.g. at least 24 hours) and a temperature (e.g. 150-204° C.), such that the material is stabilized from self-catalytic degradation.